1. Field of the Invention
This invention relates to a new process for the production of blocked polyisocyanates containing carbodiimide groups by the at least partial carbodiimidization of the free isocyanate groups in partially blocked organic polyisocyanates, to the compounds obtainable by this process and to their use as synthesis component in the production of polyurethane plastics.
2. Description of the Prior Art
Blocked isocyanates and their use as the isocyanate component, i.e. as crosslinking agent, for stoving lacquers have been known for some time (cf. for example Houben-Weyl, Methoden der Organischen Chemie, Vol. XIV/2, pages 61-70). The blocked isocyanates may be used in the production of stoving lacquers which are stable in storage both at room temperature and at mildly elevated temperatures and which only crosslink at elevated temperatures such as those in stoving ovens.
In addition, it is known that in the presence of suitable catalysts, for example phosphorus-containing catalysts, organic isocyanates form carbodiimide structures with elimination of carbon dioxide (cf. for example U.S. Pat. No. 2,941,966). In general, the diisocyanates containing carbodiimide groups thus obtained are only slightly soluble in organic solvents, as can be seen for example from DE-OS 2,436,741.
It has now surprisingly been found that polyisocyanates containing carbodiimide groups which are readily soluble in organic solvents can be obtained if the isocyanate groups of the starting polyisocyanates are partially blocked before the carbodiimidization. This solubility is surprisingly evident even in the case of solvents containing phenolic hydroxyl groups such as phenol, cresol or xylenol, although phenols of this type are known not to be inert to carbodiimide groups.
It has also been found that the products obtained by the process according to the invention containing both carbodiimide and also blocked isocyanate groups, as described in detail hereinafter, are particularly suitable polyisocyanates for stoving lacquers, particularly for wire lacquering, based on organic polyisocyanates containing blocked isocyanate groups and organic compounds containing isocyanate-reactive groups. On the one hand, the products obtained by the process according to the invention are suitable as wire lacquers which may be lacquered at particularly high speeds. On the other hand, it is possible using the products obtained by the process according to the invention to produce both tinnable and non-tinnable i.e. non-scorching, wire lacquer coatings. The difference depends upon the type of reactant used for the blocked polyisocyanates according to the invention and whether a catalyst is present since these factors affect whether just the blocked isocyanate groups or both the blocked isocyanate groups and carbodiimide groups take part in the crosslinking reaction. When reactants containing carboxyl groups are used and the reaction is carried out in the presence of a suitable metal-containing catalyst, it is possible for the carbodiimide groups to undergo crosslinking; whereas, when reactants predominantly containing hydroxyl groups are used and no catalyst is present, the reaction between hydroxyl groups and blocked isocyanate groups takes place. Non-scorching wire lacquer coatings ar obtained in the first case and tinnable wire lacquer coatings in the second case. Where uretone imine groups (formed through addition of isocyanate groups onto carbodiimide groups) are present, the isocyanate groups present therein react like blocked isocyanate groups, while the corresponding carbodiimide groups correspond in their reaction behavior to the free carbodiimide groups.